A current sensor is an electrical device which detects and measures the electrical current passing through a conductor. Current sensors are required in many applications such as energy meters, electrical generators and circuit protection systems. There are two main types of current sensors. The first type introduces a “shunt” resistance into the circuit. This involves placing a resistor in series with the conductor, and measuring the voltage across the resistor. Shunts work well for currents under a few hundred amps. For higher currents, heating occurs in the shunt which makes them difficult to manage. Furthermore, because the shunt is electrically connected to the conductor, suitable isolation methods are required to interface the shunt with processing electronics. The second type uses a magnetic transducer. Magnetic transducers work by detecting changes in the magnetic fields generated by the current carrying conductor. As such, there is no need for an electric connection with the conductor, and magnetic transducers are therefore naturally isolated from the conductor.
Two types of magnetic transducer are used for current sensing. The first type uses a highly permeable magnetic core. Such devices are susceptible to tampering, because if a permanent magnet is brought into close proximity with the device, its output can be manipulated. This is because the permanent magnet causes the permeable core to saturate, which renders the transducer insensitive to the time-varying currents it should be detecting. The second type of magnetic transducer consists of a conductive helical coil formed as a torpid. Such coils are referred to as Rogowski coils. A benefit of Rogowski coils is that they do not contain a magnetic core, and as such are immune to tampering using permanent magnets. One problem with Rogowski coils is that they are susceptible to interference from nearby current carrying conductors. As such, Rogowski coils include a so-called compensation wire. One end of the wire is passed through the coil to form a loop within the helical coil. The helical wire is for detecting currents in a conductor encircled by the toroid. The loop, or compensation wire, is for cancelling the effect external fields generated by conductors passing close to the axis of the toroid. However, even with a compensation wire, Rogowski coils can be subject to interference.